Wilson can be used for male or female. If you are concerned just call it Will. Then if someone asks you can say Wilma, William or Wilson.
I used to buy white bait and raw river prawns. I kept them in the freezer and took some out when needed. Defrost the prawn, removed the shell and chop it up into little pieces for the fish to eat. Fish and squid can be done the same. Keep it frozen until needed then defrost, cut and feed. You can use mussel, clams, virtually any marine animal. Likewise you can use any sort of fish. Just buy whatever is readily available and not going to break the bank.
If you have a group of adult banjos and they have been well looked after, then the females will be fatter from the eggs they are carrying. But you can't tell straight after feeding. If you starve them for a day then the ones that are still fat will be the girls and the slimmer ones will be the boys.
Female fish tend to eat more as well. Not sure why but it is common in most fish species. I figure they need more food to produce the eggs.